staging: omapdrm: fix crash when freeing bad fb

During unload, don't cleanup the framebuffer if it is not valid.

Signed-off-by: Andy Gross <andy.gross@ti.com>
Reviewed-by: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>